John - I've had pretty good success with rattle-can lacquer and krylon and rust-oleum clear coats (rust-oleum "clear protective enamel"). If you use the newer high-end Testors' paints (the only way to get a range of colors), be sure to use the newer Testors' clear coat as there's 'sposed to be some black magic in the pairing. I don't know of issues with better outdoor acrylics; but I like the sound of enamel better.
Be really sure to have wiped all excess goo from the water slides before top coating, as differential shrinking rates in the materials - and that goes for mix-matching clears over paints - is what will cause the decals and paints (if not perfectly cured for several days before clear-coating) to crinkle and wrinkle.
Good luck!
JB Froke
Pebble Beach, CA